I own a M4-2000 myself, and have handled (but not shot a 3100). My 2 cents. The M4-2000 has the advantage in that it can handle much higher rates of fire compared to the YHM can. The M4-2000 also is one of the "quieter" 223 cal cans available though I'm not sure how much of a real world difference it makes. My buddy has a Thunder Beast 223 TI can, and I can't hear any real difference between his and mine. On the other hand, the M4 is heavier, and the 51T mount system is known for not locking up consistently. A lot of people have had that problem including myself. And its not just a maintenance issue, my mounts (2 of them) and can did that brand new out of the box. Fortunately its fairly easy to fix, but AAC needs to fix their design on the 51T mount.